KR20190089024A - High strength galvanized steel sheet and manufacturing method thereof - Google Patents

Abstract

A high strength galvanized steel sheet capable of improving shear cross section cracking and a method of manufacturing the same. A specific component composition and a bainite having no ferrite and carbide as a total area ratio of 0 to 65%, a total area ratio of bainite having martensite and carbide of 35 to 100%, a retained austenite And a zinc plating layer formed on the base steel sheet, wherein the base steel sheet has a steel structure containing 0 to 15% by weight of a steel sheet, the amount of diffusible hydrogen in the steel sheet is 0.00008% or less (including 0% Wherein a density of gaps for dividing the entire thickness of the zinc plated layer in the cross section perpendicular to the rolling direction of the zinc plated layer is 10 / mm or more.

Description

High strength galvanized steel sheet and manufacturing method thereof

The present invention relates to a high strength galvanized steel sheet and a manufacturing method thereof, which are preferable for automotive parts.

From the viewpoint of improving the collision safety of automobiles and improving fuel economy, steel plates used for automotive parts are required to have high strength. However, the high strength of the steel sheet generally causes a deterioration in the workability. Therefore, it is required to develop a steel sheet excellent in both strength and workability. Generally, the steel sheet is sheared in the blanking line and then press-processed. Since the front end portion undergoes a large deformation, it tends to become a starting point of cracking at the time of pressing. Especially, in a high-strength galvanized steel sheet having a tensile strength (hereinafter referred to as TS) of 1000 MPa or more, this problem becomes present and there is a problem such that application parts and shapes are limited.

Patent Document 1 discloses a technique relating to a hot-dip galvanized steel sheet excellent in hole expandability by controlling the volume ratio of martensite having a plurality of different characteristics. Patent Document 2 discloses a technique relating to a hot-dip galvanized steel sheet excellent in stretch flangeability by controlling the hardness, fraction, particle diameter, and the like of martensite.

Japanese Patent Publication No. 2013-47830 Japanese Patent Publication No. 5971434

However, in Patent Documents 1 and 2, no consideration is given to the state of the diffusible hydrogen and the zinc plated layer in the base steel sheet of the coated steel sheet, and there is room for improvement.

The high strength galvanized steel sheet is required to be applied to the water part from the viewpoint of rust prevention and it is important to restrain the crack (shear end face crack) from the front end of the high strength galvanized steel sheet to strengthen the rust prevention part . It is important to make both workability and high strength compatible with this crack.

The present invention has been made in order to solve the above problems, and an object of the present invention is to provide a high strength galvanized steel sheet capable of improving shear cross-section cracking and a method of manufacturing the same.

As a result of intensive researches, the inventors of the present invention have found that, even if the steel structure is a main body of a hard body, without considering the gap between the diffusible hydrogen and the zinc plated layer in the base steel sheet, It was found that the crack became remarkable. Based on this knowledge, it is adjusted to a specific component composition, adjusted to a specific steel structure, and the concentration of diffusible hydrogen in the base steel sheet of the coated steel sheet and the concentration of hydrogen in the sheet thickness perpendicular to the rolling direction The above problems can be solved by adjusting the density of gaps for dividing the entire thickness of the zinc plated layer. The present invention has been accomplished based on these findings. More specifically, the present invention provides the following.

[1] A steel sheet comprising, by mass%, 0.05-0.30% of C, 3.0% or less of Si, 1.5-4.0% of Mn, 0.100% or less of P, 0.02% or less of S and 1.0% or less of Al Fe and inevitable impurities, ferrite and bainite having no carbide in a total area ratio of 0 to 65%, bainite having martensite and carbide in an area ratio of 35 to 100% in total, A base steel sheet having a steel structure containing residual austenite in an area ratio of 0 to 15% and having a diffusible hydrogen content of 0.00008% or less (including 0%) in a steel sheet; and a zinc A galvanized steel sheet having a plated layer and having a density of gaps of 10 / mm or more for dividing the entire thickness of the zinc plated layer in the cross section perpendicular to the rolling direction of the zinc plated layer.

[2] The high strength galvanized steel sheet according to [1], wherein the emission peak of the diffusible hydrogen is in a range of 80 to 200 占 폚.

[3] The steel sheet according to any one of the above items [1] to [3], wherein the composition further comprises 0.005 to 2.0% of Cr, 0.005 to 2.0% of Mo, 0.005 to 2.0% of V, 0.005 to 2.0% of Ni, 0.005 to 2.0% : 0.005 to 0.20%, Ti: 0.005 to 0.20%, B: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010 to 0.10%, Sn: 0.0010 to 0.50% The high strength galvanized steel sheet according to [1] or [2], which contains at least one kind of steel.

[4] The galvanized steel sheet according to any one of [1] to [3], wherein the galvanized layer is a galvannealed layer.

[5] A hot-rolled or cold-rolled sheet having the composition described in [1] or [3] is heated to an annealing temperature of 750 ° C or higher, s or more and a retention time at a temperature range of 750 ° C or higher in the heating to the cooling is 30 seconds or longer; and an annealing step after the annealing step, wherein zinc plating is performed on the annealing plate, A bending and unbending process is performed in a direction perpendicular to the rolling direction at a bending radius of 500 to 1000 mm at a temperature range of Ms to Ms-200 캜 during cooling after the zinc plating process A bending unbending process carried out at least once each time, a holding process of setting the time until the temperature reaches 100 deg. C after the bending unbending process to be at least 3 s, and cooling to 50 deg. C or less after the holding process The method of manufacturing a high strength galvanized steel sheet having a final cooling step.

[6] The method for producing a high strength galvanized steel sheet according to [5], wherein the H ₂ concentration at the annealing temperature is 30% or less in the annealing step.

[7] The method for producing a high strength galvanized steel sheet according to [5] or [6], wherein in the annealing step, the H ₂ concentration in cooling in a temperature range of 550 to 700 ° C. is 30% or less.

By using the high-strength galvanized steel sheet of the present invention, it is possible to obtain a product such as a component having excellent end-to-end cracking resistance.

Fig. 1 is a view for explaining bainite having no carbide and bainite having a carbide.
2 is an example of an image showing the gap of the plating layer.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.

<High strength galvanized steel plate>

The high strength galvanized steel sheet of the present invention has a base steel sheet and a zinc plated layer formed on the base steel sheet. First, the base steel sheet will be described, and then the zinc plating layer will be described.

The base steel sheet has a specific component composition and a specific steel structure. The base steel sheet will be described in the order of the composition of the components and the steel structure. In the description of the composition of the base metal, "%" representing the content of the component means "% by mass".

C: 0.05 to 0.30%

C is an element effective to increase the tensile strength (TS) by generating martensite or bainite containing carbide. When the C content is less than 0.05%, such an effect is not sufficiently obtained, and TS: 1000 MPa or more can not be obtained. On the other hand, if the C content exceeds 0.30%, the martensite hardens and the cracking resistance at the end of the end portion deteriorates. Therefore, the C content is set to 0.05 to 0.30%. The preferable C content for the lower limit is 0.06% or more. More preferably, it is 0.07% or more. The preferred C content for the upper limit is 0.28% or less. More preferably, it is 0.26% or less.

Si: 3.0% or less (does not include 0%)

Si is an element effective for increasing the TS by strengthening the steel. When the Si content exceeds 3.0%, the steel becomes brittle and deteriorates in cracking resistance at the end of the electric connection. Therefore, the Si content is set to 3.0% or less, preferably 2.5% or less, and more preferably 2.0% or less. The lower limit of the Si content is not particularly limited, but is preferably 0.01% or more, and more preferably 0.50% or more.

Mn: 1.5 to 4.0%

Mn is an element effective for raising TS by producing bainite containing martensite or carbide. If the Mn content is less than 1.5%, such an effect can not be sufficiently obtained, and bainite containing no ferrite or carbide which is not preferable in the present invention is produced, and TS: 1000 MPa or more can not be obtained. On the other hand, if the Mn content exceeds 4.0%, the steel becomes brittle and the cracking resistance of the electric-connection end portion deteriorates. Therefore, the Mn content is set to 1.5 to 4.0%. A preferable Mn content for the lower limit is 2.0% or more. More preferably, it is 2.3% or more. More preferably, it is at least 2.5%. The preferable Mn content for the upper limit is 3.7% or less. More preferably, it is 3.5% or less. More preferably, it is 3.3% or less.

P: not more than 0.100% (not including 0%)

P is deteriorated in the cracking resistance at the end of endurance, it is preferable to reduce the amount of P to the maximum. In the present invention, the P content can be allowed up to 0.100%. The lower limit is not specifically defined, but if it is less than 0.001%, the production efficiency is lowered, and therefore, it is preferably 0.001% or more.

S: 0.02% or less (does not include 0%)

S deteriorates the cracking resistance at the end edge, it is preferable to reduce the amount to the maximum, but in the present invention, the S content can be allowed up to 0.02%. The lower limit is not specifically defined, but if it is less than 0.0005%, the production efficiency is lowered, and therefore, it is preferably 0.0005% or more.

Al: 1.0% or less (does not include 0%)

Al acts as a deoxidizer and is preferably added at the time of deoxidation. From the viewpoint of use as a deoxidizing agent, the Al content is preferably 0.01% or more. When a large amount of Al is contained, a large amount of bainite containing no ferrite or carbide undesirable in the present invention is produced, the amount of bainite containing martensite or carbide is reduced, and the TS is not more than 1000 MPa . In the present invention, the Al content is allowed up to 1.0%. Preferably 0.50% or less.

The remainder is Fe and inevitable impurities. If necessary, however, it is preferable that 0.005 to 2.0% of Cr, 0.005 to 2.0% of Mo, 0.005 to 2.0% of V, 0.005 to 2.0% of Ni, 0.005 to 2.0% of Cu, 0.005 to 2.0% 0.001 to 0.0050%, B: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010 to 0.10%, and Sn: 0.0010 to 0.50% Or more.

Cr, Ni, and Cu are effective elements for producing martensite and bainite containing carbide and contributing to high strength. In order to obtain such an effect, it is preferable that the content is set to the lower limit value or more. On the other hand, if the content of each of Cr, Ni, and Cu exceeds the upper limit, the retained austenite tends to remain and the cracking resistance at the end portion is deteriorated. The Cr content with respect to the lower limit is preferably 0.010% or more, and more preferably 0.050% or more. The Cr content in the upper limit is preferably 1.0% or less, and more preferably 0.5% or less. With respect to the lower limit, the Ni content is preferably 0.010% or more, more preferably 0.100% or more. The Ni content in the upper limit is preferably 1.5% or less, and more preferably 1.0% or less. The Cu content with respect to the lower limit is preferably 0.010% or more, and more preferably 0.050% or more. The Cu content with respect to the upper limit is preferably 1.0% or less, and more preferably 0.5% or less.

Mo, V, Nb and Ti form carbides and are effective elements for increasing the strength by precipitation strengthening. In order to obtain such an effect, it is preferable that the content is set to the lower limit value or more. When the content of Mo, V, Nb and Ti exceeds the upper limit, the carbide is coarsened and the cracking resistance of the end portion of the present invention can not be obtained. The Mo content with respect to the lower limit is preferably 0.010% or more, and more preferably 0.050% or more. The Mo content with respect to the upper limit is preferably 1.0% or less, and more preferably 0.5% or less. The V content with respect to the lower limit is preferably 0.010% or more, and more preferably 0.020% or more. The V content with respect to the upper limit is preferably 1.0% or less, and more preferably 0.3% or less. The Nb content with respect to the lower limit is preferably 0.007% or more, and more preferably 0.010% or more. The Nb content with respect to the upper limit is preferably 0.10% or less, more preferably 0.05% or less. With respect to the lower limit, the Ti content is preferably 0.007% or more, and more preferably 0.010% or more. The Ti content is preferably 0.10% or less, more preferably 0.05% or less, with respect to the upper limit.

B is an effective element that enhances the quenching of a steel sheet, produces martensite, bainite containing carbide, and contributes to high strength. In order to obtain such an effect, the B content is preferably 0.0001% or more. More preferably, it is 0.0004% or more, and more preferably 0.0006% or more. On the other hand, if the content of B exceeds 0.0050%, the inclusions increase and the cracking resistance at the end of the end portion deteriorates. More preferably 0.0030% or less, and still more preferably 0.0020% or less.

Ca and REM are effective elements for improving the cracking resistance of the internal end portion by shape control of inclusions. In order to obtain such an effect, it is preferable that the content is set to the lower limit value or more. If the content of Ca and REM exceeds the upper limit, the amount of intervening material increases and the bending property is deteriorated. The Ca content with respect to the lower limit is preferably 0.0005% or more, and more preferably 0.0010% or more. The upper limit is preferably 0.0040% or less, more preferably 0.0020% or less. The REM content with respect to the lower limit is preferably 0.0005% or more, and more preferably 0.0010% or more. The upper limit is preferably 0.0040% or less, more preferably 0.0020% or less.

Sn and Sb are effective elements for inhibiting denitrification, debris, and the like and inhibiting the strength of steel. In order to obtain such an effect, it is preferable that the content is set to the lower limit value or more. When the content of Sn and Sb exceeds the upper limit, the cracking resistance of the electric-field end portion is deteriorated. The Sn content in the lower limit is preferably 0.0050% or more, and more preferably 0.0100% or more. The upper limit is preferably 0.30% or less, more preferably 0.10% or less. The Sb content with respect to the lower limit is preferably 0.0050% or more, and more preferably 0.0100% or more. Is preferably not more than 0.05%, more preferably not more than 0.03%, based on the upper limit.

Even if the content of Cr, Mo, V, Ni, Cu, Nb, Ti, B, Ca, REM, Sn and Sb is less than the above lower limit value, the effect of the present invention is not impaired. Therefore, when the content of these components is less than the above lower limit value, these elements are treated to be included as inevitable impurities.

In the present invention, the total amount of inevitable impurity elements such as Zr, Mg, La, and Ce may be up to 0.002%. N may be contained in an amount of 0.008% or less as an inevitable impurity.

Next, the amount of diffusible hydrogen contained in the base steel sheet of the high strength galvanized steel sheet of the present invention will be described. In the case of a plated steel sheet having a plated layer mainly composed of zinc, hydrogen which has entered from the atmosphere into the base steel sheet during reduction annealing is trapped by the subsequent plating. Diffusible hydrogen in residual hydrogen strongly influences crack propagation at the shear section, and exceeding 0.00008% significantly deteriorates cracking resistance at the end of the end section. Although this mechanism is not clear, it is believed that hydrogen in the steel reduces the energy required for crack propagation. Therefore, the amount of diffusible hydrogen in the base material steel sheet is 0.00008% or less. Preferably 0.00006% or less, and more preferably 0.00003% or less.

When the diffusion peak of the diffusible hydrogen is 80 to 200 占 폚, the hole expandability is further enhanced, among the ones satisfying the above-mentioned amount of diffusible hydrogen. Although this mechanism is not clear, it is believed that hydrogen released below 80 ° C promotes crack propagation, particularly at the end face section.

Here, the measurement of the amount of diffusible hydrogen in the steel and the emission peak of the diffusive hydrogen is carried out by the following method. A test piece having a length of 30 mm and a width of 5 mm was taken from the annealing plate and the amount of diffusible hydrogen in the steel and the emission peak of diffusible hydrogen were measured after grinding removal of the plating layer. The measurement is made by the temperature elevation desorption method, and the temperature raising rate is 200 ° C / hr. Further, the hydrogen detected at 300 DEG C or lower is referred to as diffusible hydrogen.

Next, the steel structure of the high strength galvanized steel sheet of the present invention will be described. Wherein the steel structure comprises ferrite and bainite having no carbide in a total area ratio of 0 to 65%, bainite having martensite and carbide in a total area ratio of 35 to 100%, retained austenite in an area ratio And 0 to 15%.

Total area ratio of ferrite and bainite having no carbide: 0 to 65%

Ferrite and bainite having no carbide can be appropriately contained in order to enhance the ductility of the steel sheet, but when the sum of the area ratios exceeds 65%, desired strength can not be obtained. Therefore, the sum of area ratios of ferrite and bainite free of carbide is set to 0 to 65%, preferably 0 to 50%. , More preferably 0 to 30%, and still more preferably 0 to 15%. The lower limit is preferably 1% or more. Bainite having no carbide means a plate having a thickness in parallel with the rolling direction is corroded after polishing by 3% or more, and the 1/4 position from the surface in the thickness direction of the plate is measured by SEM (scanning electron microscope) And the carbide can not be confirmed in the obtained image data. As shown in Fig. 1, in the image data, the carbide is a portion having a white point or line shape, and is distinguishable from an island-shaped martensite or retained austenite which is not a point or a line. Further, in the present invention, the case where the uniaxial length is 100 nm or less is made to be a point or a line. Here, examples of the carbide include iron-based carbides such as cementite, Ti-based carbides, Nb-based carbides, and the like. In addition, the area ratio is the value measured by the method described in the embodiment.

Total area ratio of martensite and bainite having carbide: 35 to 100%

Martensite and bainite having a carbide are necessary for obtaining the TS of the present invention. Such an effect is obtained by setting the sum of the area ratios to 35% or more. Therefore, the total area ratio of the martensite and the carbide-containing bainite is 35 to 100%. Is preferably at least 50%, more preferably at least 70%, and most preferably at least 90%, based on the lower limit. Is preferably 99% or less, more preferably 98% or less with respect to the upper limit. Bainite with carbide means a steel sheet having a thickness of 2 mm or less and having a plate thickness cross section parallel to the rolling direction is corroded with 3% or more of deviation and a 1/4 position from the surface in the thickness direction of the plate is subjected to SEM (scanning electron microscope) , And the carbide can be identified in the obtained image data. In addition, the area ratio is the value measured by the method described in the embodiment.

Area ratio of retained austenite: 0 to 15%

The retained austenite may contain 15% as an upper limit for the purpose of improving ductility or the like, but if it exceeds 15%, the cracking resistance at the end of the end portion deteriorates. Therefore, the retained austenite is 0 to 15%, preferably 0 to 12%. , More preferably 0 to 10%, and still more preferably 0 to 8%. In addition, the area ratio is the value measured by the method described in the embodiment.

As the phase other than the above, pearlite and the like can be mentioned, and up to 10% in area ratio can be allowed. That is, other than the above, the area ratio is preferably 10% or less.

Next, the zinc plated layer will be described. In the present invention, the density of gaps for dividing the total thickness of the zinc plated layer in the plate thickness cross section perpendicular to the rolling direction is 10 / mm or more.

If the gap density is less than 10 pieces / mm, hydrogen remains and the shear cross-section cracking resistance is deteriorated. Therefore, the density of the gap for dividing the entire thickness of the plating layer in the cross section perpendicular to the rolling direction of the zinc plated layer is set to 10 / mm or more. When the gap density is more than 100 pieces / mm, the powdering property is deteriorated, so that the gap density is preferably 100 pieces / mm or less. Means a gap in which both ends of the gap reach both ends in the thickness direction of the zinc plated layer. The method for measuring the gap density is as described in the embodiment.

The zinc plated layer means a layer formed by a known plating method. The zinc plated layer also includes a galvannealed layer formed by alloying treatment. The composition of the zinc plating is preferably 0.05 to 0.25% of Al and the balance of zinc and inevitable impurities.

The tensile strength of the high strength galvanized steel sheet of the present invention is 1000 MPa or more. Preferably at least 1100 MPa. The upper limit is not particularly limited, but is preferably 2200 MPa or less from the viewpoint of harmony with other properties. More preferably not more than 2000 MPa. Here, the tensile strength is a value measured by the method described in the embodiment.

The high strength galvanized steel sheet of the present invention is excellent in the cracking resistance at the end of the end portion. Specifically, the average hole expansion ratio (%) measured and calculated by the method described in the examples is 25% or more. More preferably, it is 30% or more. The upper limit of the average hole expansion ratio (%) is not particularly limited, but is preferably 70% or less from the viewpoint of harmony with other properties. More preferably, it is 50% or less.

&Lt; Method of producing high strength galvanized steel sheet &gt;

The method for manufacturing a high strength galvanized steel sheet of the present invention has an annealing step, a zinc plating step, a bending unbend step, a stay step, and a final cooling step. The temperature is based on the surface temperature of the steel sheet.

In the annealing step, the hot-rolled sheet or the cold-rolled sheet is heated to an annealing temperature of 750 ° C or higher, and the region of 550 to 700 ° C is cooled at an average cooling rate of 3 ° C / s or more. And the residence time in the temperature range is 30 seconds or more.

The method for producing the hot-rolled sheet or the cold-rolled sheet as a starting material is not particularly limited. In order to prevent macro segregation, the slab used in the production of the hot-rolled sheet or the cold-rolled sheet is preferably manufactured by the continuous casting method, and the slab may be produced by the roughing method or the thin slab casting method. In order to hot-roll the slab, the slab may be once cooled to room temperature, and then reheated and hot-rolled. Alternatively, the slab may be charged into a furnace without being cooled to room temperature to perform hot rolling. Or an energy saving process in which hot rolling is performed immediately after a slight boiling heat is applied. In the case of heating the slab, it is preferable to heat the slab to a temperature of 1100 DEG C or higher in order to dissolve the carbide or prevent an increase in the rolling load. In order to prevent the scale loss from increasing, the heating temperature of the slab is preferably 1300 DEG C or less. The slab heating temperature is the temperature of the slab surface. When hot rolling the slab, the rough bar after rough rolling may be heated. A so-called continuous rolling process in which the rough bars are joined together and finish rolling is continuously performed can be applied. Finish rolling may increase the anisotropy and lower the workability after cold rolling and annealing, and therefore it is preferable to carry out the finish rolling at a finishing temperature equal to or higher than the A _r3 transformation point. In order to reduce the rolling load and to make the shape and the material uniform, it is preferable to carry out lubrication rolling in which the coefficient of friction is 0.10 to 0.25 in the entire or partial pass of the finish rolling. The steel sheet wound after hot rolling is subjected to heat treatment and cold rolling as necessary after the scale is removed by pickling or the like.

The heating temperature (annealing temperature) is 750 ° C or higher. When the annealing temperature is less than 750 캜, the formation of austenite becomes insufficient. Since austenite produced by annealing becomes martensite or bainite (including both carbides and not) in the final structure by bainite transformation or martensite transformation, the formation of austenite If it is insufficient, desired steel structure can not be obtained in the steel sheet. Therefore, the annealing temperature should be 750 ° C or higher. Although the upper limit is not particularly specified, it is preferably 950 DEG C or less from the viewpoint of operability and the like.

It is preferable that the H ₂ concentration at the annealing temperature in the annealing step is 30% (volume%) or less. Thereby, the amount of hydrogen entering the steel sheet can be further reduced, and the cracking resistance at the end of the end portion can be further improved. More preferably, it is 20% or less.

The average cooling rate in the region of 550 to 700 占 폚 is set to 3 占 폚 / s or more. When the average cooling rate in the region of 550 to 700 占 폚 is less than 3 占 폚 / s, a large amount of bainite containing no ferrite or carbide is generated, and a desired steel structure is not obtained. Therefore, the average cooling rate in the region of 550 to 700 占 폚 is 3 占 폚 / s or more. The upper limit is not specifically defined, but is preferably 500 ° C / s or less from the viewpoint of operability and the like.

The H ₂ concentration in the cooling in the temperature range of 550 to 700 ° C is preferably 30% (volume%) or less. When this condition is satisfied, the diffusible hydrogen released at a low temperature is reduced and the cracking resistance at the end of the end portion can be further improved. More preferably, it is 20% or less.

The cooling stopping temperature of the cooling is not particularly limited, but is preferably 350 to 550 DEG C because it is necessary to contain austenite after galvanization or after alloying.

Between the heating and the cooling, the residence time at a temperature of 750 ° C or higher is 30 seconds or more. When the residence time is less than 30 seconds, the formation of austenite becomes insufficient, and a desired steel structure can not be obtained in the steel sheet. Therefore, it should be 30 seconds or more. Although the upper limit is not particularly specified, it is preferably 1000 seconds or less from the viewpoint of operability and the like.

After the cooling, the reheating may be performed for 1 to 100 seconds at a holding temperature in the temperature range of the heating temperature Ms to 600 ° C. When reheating is not carried out, it may be maintained at the cooling stop temperature, and the holding time at the cooling stop temperature is preferably 250 seconds or less. More preferably 200 seconds or less. The lower limit is preferably 10 seconds or more, and more preferably 15 seconds or more.

The temperature and time conditions until plating are not specifically defined. However, since it is necessary to contain austenite after galvanization or after alloying, the temperature up to the plating is preferably 350 DEG C or higher.

The galvanizing step is a step of performing galvanization on an annealing plate after the annealing step and further performing alloying treatment as necessary. For example, it is preferable that the alloy contains 0 to 20% of Fe, 0.001 to 1.0% of Al, and at least one of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Ti, Be, Bi, and REM in a total amount of 0 to 30%, and the balance of Zn and inevitable impurities is formed on the surface of the cooled annealing plate.

The plating method is not particularly limited, and general methods such as hot dip galvanizing and electro-galvanizing may be employed, and the conditions may be suitably set. Alternatively, the galvannealing may be performed by heating after hot dip galvanizing. The heating temperature for the alloying treatment is not particularly limited, but is preferably 460 to 600 ° C.

The bending unbending process is a process in which the bending and unbending processes are performed one or more times at a bending radius of 500 to 1000 mm in a direction perpendicular to the rolling direction in a temperature range of Ms to Ms-200 DEG C during cooling after the zinc plating process .

A gap (a gap for dividing the total thickness of the zinc plated layer) is formed through the entire thickness of the zinc plated layer in order to alleviate the residual stress due to the difference in expansion ratio between the zinc plated layer and the base steel sheet after the galvanizing or cooling after galvanizing. At this time, if austenite is contained, the expansion due to the martensite transformation occurs when the Ms point or less is reached, and the formation of the gap in the zinc plating layer can be adjusted. Also, by controlling the tension applied to the surface by bending, the formation of gaps in the zinc plated layer can be adjusted. By performing these bending and unbending processes one or more times (preferably 2 to 10 times) at a bending radius of 500 to 1000 mm at the temperature range of Ms to Ms - 200 ° C in the above range, It is possible to adjust the gap density of the zinc plated layer in the desired range. The bending angle is preferably in the range of 60 to 180 degrees. If either the temperature range, the bending radius, or the bending process water is outside the specified range, the desired gap density can not be obtained, and the amount of hydrogen emission in the subsequent cooling process is reduced and the cracking resistance at the end portion is deteriorated. The bending unbending process is required to be performed over the entire plate. For bending unbending, it is preferable that the bending unbending process is performed over the entire plate by a roll at the time of carrying the steel plate. The Ms point is the temperature at which the martensitic transformation starts, and is determined by a foramaster.

The retention process is a process for setting the time from the bending unbending process to 100 占 폚 for 3 s or more.

When the time from the bending unbending to 100 deg. C is set to 3 s or more, hydrogen is released from the gap of the plating formed by bending unbending, and excellent excellent end cracking resistance is obtained. Bending unbending is also the first bending unbending performed below the Ms point.

The final cooling step is a step of cooling the holding step to 50 DEG C or lower. The cooling to 50 ° C or lower is necessary for the subsequent oil coating. The cooling rate in the cooling is not particularly limited, but usually the average cooling rate is 1 to 100 ° C / s.

After the cooling, temper rolling or further bending unbending may be performed.

Example

A steel having the constituent composition shown in Table 1 was melted by a converter and turned into a slab by a continuous casting method, followed by heating to 1200 DEG C, followed by pressure reduction and finish rolling to obtain a hot rolled steel sheet having a thickness of 3.0 mm. The hot rolling was carried out at a finish rolling temperature of 900 캜 and a coiling temperature of 500 캜. Subsequently, after the pickling, a cold-rolled sheet having a thickness of 1.4 mm was cold-rolled to prepare a cold-rolled sheet and provided for annealing. The annealing was carried out by a continuous hot-dip galvanizing line under the conditions shown in Table 2 to prepare hot-dip galvanized steel sheets and galvannealed hot-dip galvanized steel sheets 1 to 38. Here, the galvanized steel sheet (GI) is immersed in a plating bath at 460 DEG C to form a coating with an adhesion amount of 35 to 45 g / m &lt; 2 &gt;, and the galvannealed steel sheet (GA) lt; RTI ID = 0.0 &gt; s. &lt; / RTI &gt; The obtained coated steel sheet was subjected to bending unbending processing under the conditions shown in Table 2. Further, any bending unbending was performed by bending unbending the entire plate by a roll. After the bending unbending process, the retention process was performed under the conditions shown in Table 2, and then cooled to 50 DEG C or less. Tissue observation, tensile properties, diffusible hydrogen content, hydrogen release peak temperature and cracking resistance at the end portions were evaluated according to the following test methods.

Tissue observation (area ratio of each phase)

The area ratio of ferrite, martensite, and bainite refers to the ratio of the area of each structure occupying in the observation area. These area ratios are obtained by cutting a sample from the steel sheet after annealing and polishing the plate thickness section parallel to the rolling direction Then, the film was corroded with 3% detachment from the surface, and 1/4 position in the plate thickness direction from the surface was photographed at a magnification of 1500 times with SEM (scanning electron microscope) at 3 times each, and image data obtained from Media Cybernetics -Pro is used to calculate the area ratio of each tissue, and the average area ratio of the field of view is defined as the area ratio of each tissue. In the image data, the ferrite is black, the martensite and the residual austenite are white or light gray, the bainite is carbide or island-shaped martensite whose orientation is aligned, or black or dark gray Since the grain boundary can be confirmed, it is possible to distinguish between bainite containing no carbide and bainite containing carbide. As shown in Fig. 1, in the image data, white or light gray portions ). In the present invention, the area ratio of bainite is the area ratio of the black or dark gray color portion of the bainite excluding the white or light gray portion. The area ratio of the martensite was determined by subtracting the area ratio of the retained austenite, which will be described later (volume ratio is regarded as area ratio), from the area ratio of the white or light gray structure. Further, in the present invention, the martensite may be an auto-tempered martensite or a tempered martensite containing carbide. In addition, martensite containing carbide is not aligned carbide and is different from bainite. The island-shaped martensite is also a martensite having any of the above characteristics. In the present invention, white or non-pointed white portions are distinguished as martensite or retained austenite. In the present invention, pearlite can be distinguished as a black and white layered structure in some cases although it may not be contained.

The volume percentage of retained austenite was obtained by grinding the steel sheet after annealing to 1/4 of the plate thickness, using a Kα line of Mo with an X-ray diffractometer on a surface further polished by chemical polishing by 0.1 mm, the integral reflection intensity of the (200), (211) and (220) planes of the fcc iron (austenite), the (220) plane and the (311) plane and the bcc iron (ferrite) , and the volume ratio was calculated from the intensity ratio of the integral reflection intensity from each side of fcc iron to the integral reflection intensity from each side of bcc iron. The volume ratio is regarded as the area ratio.

In the table, &quot; V (F + B1) &quot; means the total area ratio of ferrite and bainite containing no carbide, &quot; V (M + B2) &quot; means bainite containing martensite and carbide "V (γ)" means the area ratio of the retained austenite, and other means the area ratio of the other phases.

Tissue observation (gap density)

SEM image of the vicinity of the surface layer was taken at a magnification of 3000 times at a field of 30, and the void density was divided by the surface line length of the entire steel sheet to divide the entire thickness of the plating present in the field of view so as to obtain a gap density of at least 10 pieces / . An example of the photographed image is shown in Fig.

The amount of diffusible hydrogen in the steel, the emission peak of diffusible hydrogen

A test piece having a length of 30 mm and a width of 5 mm was taken from the annealing plate and the amount of diffusible hydrogen in the steel and the emission peak of diffusible hydrogen were measured after grinding removal of the plating layer. The measurement was made by the temperature elevation desorption method, and the heating rate was 200 ° C / hr. Further, the hydrogen detected at 300 DEG C or lower was made a diffusible hydrogen. The results are shown in Table 3.

Tensile test

JIS No. 5 tensile test specimen (JIS Z 2201) was taken from the annealing plate in a direction perpendicular to the rolling direction and subjected to a tensile test in accordance with JIS Z 2241 with a deformation rate of 10-3 / s. Respectively. In the present invention, 1000 MPa or more is acceptable.

Cracking endurance

Shear cracking was evaluated by hole expansion test. Test specimens having a length of 100 mm and a width of 100 mm were taken from the annealing plate and basically, the hole expansion test was conducted three times in accordance with JFST 1001 (Steel Federation Standard) to obtain an average hole expansion ratio (%), And the end cracking property was evaluated. However, the clearance was set to 9%, and a large amount of the cross section was formed on the cross section. In the present invention, 25% or more is acceptable.

The results are shown in Table 3.

In the present invention, all of them are high-strength steel sheets having excellent internal-end cracking resistance. On the other hand, in the comparative example deviating from the scope of the present invention, the desired strength was not obtained or the cracking resistance of the electric end portion was not obtained.

Industrial availability

According to the present invention, it is possible to obtain a high-strength galvanized steel sheet having a TS of 1000 MPa or more and having excellent cracking resistance at the end portion of the internal end. The use of the high-strength member and the high-strength steel sheet of the present invention for use in automobile parts can greatly contribute to improvement of collision safety of automobiles and improvement of fuel economy.

Claims (7)

In terms of% by mass,
C: 0.05 to 0.30%
Si: 3.0% or less,
Mn: 1.5 to 4.0%
P: not more than 0.100%
S: 0.02% or less,
Al: 1.0% or less, the balance being Fe and inevitable impurities,
The ferrite and the bainite having no carbide are added in an amount of 0 to 65% in total, the bainite having martensite and carbide in a total area ratio of 35 to 100%, the retained austenite in an area ratio of 0 to 15% Having a steel structure,
A base steel sheet in which the amount of diffusible hydrogen in the steel sheet is 0.00008% or less (including 0%) in mass%
And a zinc plating layer formed on the base steel sheet,
Wherein a density of gaps for dividing the entire thickness of the zinc plated layer in the cross section perpendicular to the rolling direction of the zinc plated layer is 10 / mm or more. The method according to claim 1,
And the emission peak of the diffusible hydrogen is in a range of 80 to 200 占 폚. 3. The method according to claim 1 or 2,
The composition of the above-mentioned components is further, in mass%
0.005 to 2.0% of Cr,
Mo: 0.005 to 2.0%
V: 0.005 to 2.0%
Ni: 0.005 to 2.0%
Cu: 0.005 to 2.0%
Nb: 0.005 to 0.20%
Ti: 0.005 to 0.20%
B: 0.0001 to 0.0050%,
Ca: 0.0001 to 0.0050%,
REM: 0.0001 to 0.0050%,
Sb: 0.0010 to 0.10%,
And Sn: 0.0010 to 0.50% based on the total weight of the high-strength galvanized steel sheet. 4. The method according to any one of claims 1 to 3,
Wherein the zinc plated layer is a galvanized zinc plated layer. A hot-rolled or cold-rolled sheet having the composition described in claim 1 or 3 is heated to an annealing temperature of 750 ° C or higher and held as required. After that, The annealing step in which the residence time in the temperature range of 750 DEG C or more in the heating to the cooling is 30 seconds or more;
A galvanizing step of performing galvanization on the annealing plate after the annealing step and further performing an alloying treatment if necessary,
A bending unbending process in which a bending and unbending process is performed at least once in a bending radius of 500 to 1000 mm in a direction perpendicular to the rolling direction in a temperature range of Ms to Ms-200 ° C during the cooling after the zinc plating process and,
A holding step of keeping the time until the temperature reaches 100 deg. C after the bending unbending process for 3 s or more,
And a final cooling step of performing cooling to 50 DEG C or lower after the retention process. 6. The method of claim 5,
Wherein the H ₂ concentration at the annealing temperature in the annealing step is 30% by volume or less. The method according to claim 5 or 6,
Wherein the H ₂ concentration in cooling in a temperature range of 550 to 700 ° C is 30% by volume or less in the annealing step.

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